1. Field of the Invention
The present invention relates to a read circuit and a read method using a self-reference sensing technique that are employed particularly in a magnetic random access memory (MRAM) utilizing the magnetoresistive effect.
2. Description of the Related Art
A magnetic random access memory is a memory device that includes magnetoresistive devices as memory cells. Despite being a nonvolatile memory, a magnetic random access memory exhibits high integration, high reliability, low power consumption, and high-speed performance. Therefore, such a magnetic random access memory is expected to be one of the next-generation memory devices.
The magnetoresistive effects are divided into two types: GMR (giant magnetoresistive) effect and TMR (tunneling magnetoresistive) effect.
The “GMR effect” is the phenomenon that causes the resistance of a conductor interposed between two ferromagnetic layers to vary with the spin directions of the two ferromagnetic layers. However, the MR (magnetoresistive) ratio representing the varying ratio of the magnetic resistance with the GMR effect is as low as 10%, and accordingly, read signals become very small.
Therefore, to realize a magnetic random access memory utilizing the GMR effect, it is necessary to secure great read margins. In reality, however, great read margins are not easily secured, and for this reason, a magnetic random access memory utilizing the GMR effect is considered to be inadequate for practical use.
The “TMR effect” is the phenomenon that causes the junction resistance of a ferromagnetic tunnel junction formed with two ferromagnetic layers and an insulating layer (a tunnel barrier layer) interposed between the two ferromagnetic layers, i.e., the tunnel conductance of the insulating layer, to vary with the cosine of the relative angle of the magnetization of the two ferromagnetic layers.
Typical examples using the TMR effect include MTJ (magnetic tunnel junction) devices using magnetoresistance variations caused by a spin-deflecting tunneling effect. In a MTJ device, the tunnel probability becomes the highest and the junction resistance of the ferromagnetic tunnel junction becomes the smallest, if the spin directions of the two ferromagnetic layers are the same (or parallel to each other). If the spin directions of the two ferromagnetic layers are opposite to each other (or anti-parallel to each other), the tunneling probability becomes the lowest and the junction resistance of the ferromagnetic tunnel junction becomes the greatest.
To arbitrarily put the spin directions in one of the above two states, one of the two ferromagnetic layers is normally fixed in one direction of magnetization and serves as a pin layer (a fixed layer) that is not affected by an external magnetic field. Meanwhile, the other one of the two ferromagnetic layers is designed to be a free layer (a memory layer) that can have a magnetizing direction made parallel or anti-parallel to the direction of magnetization of the pin layer in the existence of an external magnetic field. With the TMR effect, the MR ratio that represents the varying ratio of a magnetoresistance can be made 50% or higher, and thus, reliable read operations can be performed.
Accordingly, magnetoresistive devices utilizing the TMR effect are normally employed in the development of magnetic random access memories at present.
Meanwhile, a self-reference sensing technique has been developed for use in magnetic random access memories, so as to achieve reliable read operations that are not adversely affected by variations in the characteristics of memory cells (see “0.24 μm 2.0V 1T1MTJ 16 Kb NV Magnetoresistive RAM with Self Reference Sensing”, ISSCC Digest of Technical Papers, 2003).
This technique is characterized in that two read operations are performed on a designated cell to be read, and the value of the data stored in the designated cell is determined by comparing the data read through the two read operations.
For example, after the first read operation is performed on the designated cell, “0” is written in the designated cell, and the second read operation is then performed on the designated cell. If the data (signal) read out through the two read operations are the same, the value of the data stored in the designated cell is determined to be “0”. If the data read out through the two read operations differ from each other, the value of the data stored in the designated cell is determined to be “1”.
The determination of the value of the data stored in the designated cell is performed by detecting the difference between the signal obtained through the two read operations. For example, a read current I1 is applied to the designated cell in the first read operation, and a read current I2 (>I1) is applied to the designated cell in the second read operation.
By the conventional self-reference sensing technique, however, each of the margins allowed for the “0”/“1” determination to be performed by an operational amplifier is as small as half of the greatest signal difference obtained with the MR ratio of the corresponding magnetoresistive device.
Only such small margins are allowed, because guard bands or offsets (the read currents I1 and I2) need to be set so as to determine whether there is not a difference between the signal obtained through the two read operations (whether the data values are the same). Therefore, there is an increasing demand for a technique to eliminate this drawback.